Cellar Insulation

Would anyone be kind enough to tell me the best way to insulate my cellar?
Its 350 sq ft, with air vents (grills) at the front, the back and where the old coal shute was. The ground floor floorboards have gaps between them enough to see the floor below.

My plan was this. Seal up the gaps with plastic wood, buy some fibreglass matting stuff cut it into strips and staple/ tack it betweenthe beams. Then I'd nail up some board over the beams. Not sure what board to use though. Does this seem a good plan?

Thanks.

In my cellar there is ordinary fibreglass insulation between the joists held up with plastic sheeting stapled to the underside of the joists. We never had any problems with this. You would not need to fill the gaps between the floorboards if you did this and to be honest I'm not sure that filler between floorboards would stay put for very long.

Thanks very much. Much appreciated.

polystyrene is a good insulator